The present invention generally relates to methods for sensing structural features on a moving fastener tape during the automated manufacture of reclosable packaging. In particular, the invention relates to methods for sensing deformations or attached articles on a plastic fastener tape of the type comprising a pair of mutually interlocked zipper strips, the latter having a length sufficient to form multiple package-length zippers when processed on a bag making machine, thermoformed packaging machine, form-fill-seal machine or other machine for making reclosable packaging.
During the automated manufacture of reclosable packages, typically a thermoplastic fastener tape unwound from a supply reel or spool is joined (e.g., by conductive heat sealing) to a web of thermoplastic packaging material. The web-to-fastener tape sealing operation can be performed either intermittently (i.e., during dwell times interspersed between intermittent advancements) or continuously (i.e., while the fastener tape and web are advancing continuously).
In cases where a fastener tape without pre-sealing and without sliders must be joined with a web of packaging material, there is a need for the fastener tape to be properly aligned with the web of film (i.e., straightness and cross-machine alignment), but there is no need to register the fastener tape relative to the web in a machine direction. This is due to the fact that the fastener tape has a constant profile along its length and thus has no structural features that need to be registered relative to respective package-length sections of the web of packaging material.
The fastener tape typically comprises a pair of continuous zipper strips, each zipper strip having a respective constant profile produced by extrusion. Typically, the respective zipper strip profiles have complementary shapes that allow the zipper strips to be interlocked. These closure profiles may be of the rib-and-groove variety, the interlocking-hook variety or any other suitable fastenable structures. Pre-sealing of the fastener tape involves crushing and fusing the closure profiles and, if the zipper strips are flanged, the zipper flanges as well at spaced intervals along the fastener tape at locations where the fastener tape will be ultimately cut when each finished package is severed from the work in process. Pre-sealing may be accomplished by ultrasonic stomping or thermal crushing. In cases where the fastener tape is pre-sealed before entering the packaging machine, it is desirable that the midplane of each pre-seal be registered within a maximum allowable deviation relative to a corresponding cut line.
In cases where sliders are inserted at spaced intervals along the fastener tape before the latter enters the packaging machine, it is common to combine the joinder of the closure profiles at spaced intervals with the formation of slider end stop structures on the fastener tape. Although slider end stops can be attached to or inserted on the fastener tape, it is common practice to simply deform and fuse the thermoplastic material of the closure profiles strips wherever slider end stops are needed. Typically, the zipper material is softened by applying ultrasonic wave energy and the thus-softened zipper material is shaped to form a slider end stop structure. If the zipper has flanges, the zipper flanges can be fused during the same ultrasonic stomping operation or during a separate thermal crushing operation. The slider end stop structure, when bisected by a cut line, will form back-to-back slider end stops for adjacent packages. The slider end stop structure is formed at a location such that its midplane will be generally coplanar with the plane of cutting when the finished package is severed from the work in process. Thus, it is important that the midplane of each slider end stop formation on the fastener tape be registered within a maximum allowable deviation relative to a corresponding cut line.
During the initial setup of a machine that joins a fastener tape to a web or webs of packaging material, the midplane of a leading pre-seal or slider end stop structure may be manually aligned with the cutting blade that severs the completed package from the work in process. There is a need for means to ensure that each subsequently formed pre-seal or slider end stop structure will ultimately arrive at a position whereat its midplane will also be generally aligned with the cutting blade. One method of accomplishing the foregoing involves the step of sensing or detecting the passage of each pre-seal or slider end stop structure (or each slider) at a fixed location during fastener tape advancement. This information is then used to adjust the distance by which the fastener tape is advanced in the interval between successive pre-sealing or slider end stop formation (with concurrent slider insertion) operations.
Thus, there is a need for an accurate and reliable method for sensing or detecting a repeating structural feature formed on or attached to a moving fastener tape as it passes a fixed location in a packaging machine.